Shower/tub temp. not hot enough

Hi,I'm having trouble getting hot water in my upstairs tub/shower-It's lukewarm at best and it's good and hot everywhere else in the house including the sink in the same bathroom.The tub/shower control is a Symmons Temptrol unit that was installed in 1988.I thought that the problem would be in the spindle unit(I have well water and I had to replace this once before)but it did not help things.Also,I tried adjusting the anti-scald set-screw(I backed it way out so it is not even contacting)but this did not help either.What else might it be?A clogged pipe?Thanks very much for any advice.

http://www.symmons.com/Customer-Service.aspx

 

a partly blocked hot water line could be the problem. I would really suspect the old valve. Replace with a Delta and you will probably be fine.